Method for manufacturing articles for high temperature use, and articles made therewith

ABSTRACT

A method for manufacturing an article for use in a high-temperature environment, and an article for use in such an environment, are presented. The method comprises providing a substrate; selecting a desired vertical crack density for a protective coating to be deposited on the substrate; providing a powder, wherein the powder has a size range selected to provide a coating having the desired vertical crack density; and applying a thermal-sprayed coating to the substrate, the coating having the desired vertical crack density, wherein the powder is used as a raw material for the coating.

FEDERAL RESEARCH STATEMENT

This invention was first conceived or reduced to practice in theperformance of work under contract DEFCO2OOCH11047 with the UnitedStates Department of Energy. The United States of America may havecertain rights to this invention.

BACKGROUND OF INVENTION

This invention relates to protective coatings for use in hightemperature environments. More particularly, this invention relates tomethods for manufacturing coated articles for use in high-temperatureenvironments. This invention also relates to articles for use in suchenvironments.

Materials used in high-temperature applications such as, for example,gas turbine engines, heat exchangers, internal combustion engines, andthe like, are typically subjected to a potentially detrimentalcombination of heat, oxidative gas mixtures (including air, forinstance), and other deleterious species such as water vapor. Protectivecoatings are often applied to component surfaces to prolong servicelives and enhance performance. Thermal barrier coatings (TBC's) andenvironmental barrier coatings (EBC's) are two examples of protectivecoatings used in these and other applications. TBC's often comprise alayer of material, such as, for example, yttria-stabilized zirconia,having low thermal conductivity and high resistance to hot gas. EBC'sare often used where the operating environment of a machine is corrosiveor otherwise chemically detrimental to the materials comprising the bulkof key components. For example, U.S. Pat. No. 6,387,456 describes amethod for applying an EBC to silicon based substrate materials, whichhave been proposed for use in high temperature applications. The EBC inthis work comprised a barium-strontium aluminosilicate (BSAS) barrierlayer, which protects the silicon-bearing substrate from environmentscontaining high-temperature water vapor. Intermediate layers are alsooften applied between the substrate and EBC or TBC to provide enhancedadhesion, oxidation resistance, thermal expansion match, and the like.

EBC's and TBC's, which are generally ceramic materials, are oftenapplied using thermal spray methods, including, for example, air plasmaspraying, vacuum plasma spraying, and high velocity oxy-fuel (HVOF)spraying. The application of a high-performance, protective EBC or TBCpresents a significant technical challenge, because the microstructureand properties of the finished coatings depend in large part on theprocess parameters used during the thermal spray application of thecoating material. Such coatings are often required to possess a balanceof competing properties such as, for example, high density and hightolerance for strain. Selection and control of thermal spray processparameters is thus an important aspect of the manufacture and repair ofarticles for use in high-temperature applications.

Therefore, there is a need to provide methods to control themicrostructure and properties of protective thermal sprayed coatings, inorder to improve the service lives and performance of high-temperaturecomponents. There is a further need for articles having enhancedperformance in high-temperature applications.

SUMMARY OF INVENTION

Embodiments of the present invention address these and other needs. Oneembodiment is a method for manufacturing an article for use in ahigh-temperature environment, the method comprising providing asubstrate; selecting a desired vertical crack density for a protectivecoating to be deposited on the substrate; providing a powder, whereinthe powder has a size range selected to provide a coating having thedesired vertical crack density; and applying a thermal-sprayed coatingto the substrate, wherein the powder is used as a raw material for thecoating. The coating has the desired vertical crack density.

A second embodiment is an article for use in high temperatureenvironments, the article comprising a substrate comprising silicon; andan environmental barrier coating comprising barium aluminosilicate,wherein the coating comprises a vertical crack density of greater thanabout 4 cracks per linear centimeter.

DETAILED DESCRIPTION

In the method embodiments of the present invention, a substrate isprovided. In certain embodiments, the provided substrate comprises amaterial that comprises silicon, for example, at least one of asilicon-containing ceramic and a silicon-containing metal alloy.Examples of suitable ceramics include, but are not limited to,fiber-reinforced composite materials, such as composites comprisingsilicon carbide. In certain embodiments, providing the substrate furthercomprises providing a component of a gas turbine assembly, and manysilicon-containing materials are susceptible to degradation at thetemperatures commonly used for such applications, where the environmentcontains water vapor, thus the application of protective barriercoatings is desirable.

A desired vertical crack density is selected for the protective coatingto be deposited on the substrate. The presence of vertical cracks (thatis, cracks running along the entire cross-sectional thickness of acoating from a first interface at the deposition surface to a second,opposing interface) in thermal sprayed coatings advantageously enhancesthe compliance of the coating, allowing the coating to tolerate higheramounts of strain before failure than dense coatings of the samematerial. However, vertical cracks detract from the ability of thecoating to isolate the substrate from the environment, thus reducing theprotective properties of, for example, an EBC. Therefore, those skilledin the art will appreciate that the selection of a desired verticalcrack density is based upon an optimization of competing properties,such as, for example, compliance versus barrier effectiveness. Incertain applications, the strain tolerance of an EBC may be highlyimportant, to the point where its protective properties may besacrificed somewhat to attain higher compliance. In such cases, acertain allowable range of vertical cracks may be specified. One methodin the art for quantifying vertical cracks is to examine a given lengthof cross-sectioned, coated substrate; count the number of verticalcracks present in this known length, and calculate the ratio of numberof cracks per unit length of cross section. This ratio is referred toherein as “vertical crack density.” In some embodiments, selecting thedesired vertical crack density comprises selecting a vertical crackdensity from about 0 cracks per linear centimeter of coating to about 30cracks per linear centimeter of coating. In certain embodiments,selecting the vertical crack density comprises selecting a verticalcrack density from about 0 cracks per linear centimeter of coating toabout 12 cracks per linear centimeter of coating.

The method of the present invention further comprises providing apowder. As used herein, the term “powder” refers to a population ofindividual particles, wherein each individual particle has a diameter,also referred to herein as a “particle size.” The powder, as apopulation, has a statistical distribution of individual particle sizes(“particle size distribution”). According to embodiments of the presentinvention, powder is provided, wherein the powder has a particle sizedistribution selected to provide a thermal spray coating having theaforementioned desired vertical crack density. The present inventorshave discovered that by controlling the particle size distribution ofthe powder used as raw material in a thermal spray process, the verticalcrack density of the resultant coating can be controlled over a widerange, including crack densities down to about 0 cracks per linearcentimeter. The ability to control coating microstructure by controllingparticle size distribution gives an additional degree of freedom in thethermal spray processing of ceramic barrier coatings, including EBC'sand TBC's.

The present inventors discovered that the vertical crack density of athermal sprayed barrier coating, such as an EBC, is related to theamount of relatively small-diameter particles (particles with diametersless than about 44 micrometers) in the powder used to process thecoating in general, for a given set of thermal spray processingconditions, powders with fewer small-diameter particles yield coatingswith lower vertical crack densities. Thus, according to embodiments ofthe present invention, the vertical crack density of a thermal sprayedbarrier coating can be controlled by specifying a particular sizedistribution of the powder used to process the coating.

Particle size distributions are often characterized in the industry bydetermining values for particle diameters at certain points in thecumulative particle size distribution of the powder. For example, aparticle size distribution may be specified by reporting the diameterscorresponding to the tenth, fiftieth, and ninetieth percentile points inthe cumulative particle size distribution; illustratively, about tenpercent of the powder by volume therefore comprises particles havingdiameters less than this tenth percentile diameter value.

Embodiments of the present invention include instances wherein providingthe powder comprises providing a powder having a particle sizedistribution, wherein about 10 volume percent of the powder comprisesparticles having diameters less than a tenth percentile diameter valuein the range from about 25 micrometers to about 50 micrometers, about 50volume percent of the powder comprises particles having diameters lessthan a fiftieth percentile diameter value in the range from about 50micrometers to about 70 micrometers, and about 90 volume percent of thepowder comprises particles having diameters less than a ninetiethpercentile diameter value in the range from about 85 micrometers toabout 105 micrometers.

In certain embodiments, providing the powder comprises providing apowder having a particle size distribution wherein the tenth percentilediameter value is in the range from about 25 micrometers to about 30micrometers, the fiftieth percentile diameter value is in the range fromabout 50 micrometers to about 55 micrometers, and the ninetiethpercentile diameter value is in the range from about 85 micrometers toabout 90 micrometers. This size distribution corresponds, for example,to powders that are presently commercially available (herein referred toas “baseline grade powder”) for thermal spray processing ofenvironmental barrier coatings comprising barium strontiumaluminosilicate.

In alternative embodiments, the size distribution of the powder isgenerally coarser than the aforementioned baseline grade. The coarserpowder may be obtained, for example, by passing baseline grade powderthrough at least one sieve, by selective air classification, or by othermethods commonly used in the art. According to these embodiments,providing the powder comprises providing a powder having a sizedistribution wherein the tenth percentile diameter value is in the rangefrom about 35 micrometers to about 50 micrometers, the fiftiethpercentile diameter value is in the range from about 55 micrometers toabout 70 micrometers, and the ninetieth percentile diameter value is inthe range from about 85 micrometers to about 105 micrometers.

In certain embodiments pertinent to the use of coarser powder, providingthe powder comprises providing a powder (herein referred to as class Apowder) having a particle size distribution wherein the tenth percentilediameter value is in the range from about 35 micrometers to about 45micrometers, the fiftieth percentile diameter value is in the range fromabout 55 micrometers to about 65 micrometers, and the ninetiethpercentile diameter value is in the range from about 85 micrometers toabout 90 micrometers. In other embodiments, providing the powdercomprises providing a powder (herein referred to as class B powder)having a particle size distribution wherein the tenth percentilediameter value is in the range from about 45 micrometers to about 50micrometers, the fiftieth percentile diameter value is in the range fromabout 60 micrometers to about 70 micrometers, and the ninetiethpercentile diameter value is in the range from about 95 micrometers toabout 105 micrometers. As described above, the use of powder, such as,for example, class B powder, having particle size distributions that arecoarse relative to the aforementioned baseline grade generally result incoatings having relatively lower vertical crack densities, when used inaccordance to embodiments of the present invention.

In some embodiments, providing the powder comprises providing a materialcapable of forming an environmental barrier layer to protect thesubstrate from a high temperature, chemically aggressive environment;examples of such a material include, for example, a ceramic material. Inparticular embodiments, providing the ceramic material comprisesproviding a material comprising barium strontium aluminosilicate (BSAS),the aforementioned material commonly used to manufacture EBC's toprotect silicon-containing substrates from high temperature environmentscontaining water vapor. In alternative embodiments, providing the powdercomprises providing a material, such as, for example, a ceramic, capableof forming a thermal barrier layer to protect the substrate from a hightemperature environment. In particular embodiments, providing theceramic material comprises providing a material comprisingyttria-stabilized zirconia.

A thermal-sprayed coating is applied to the substrate, wherein theaforementioned powder is used as a raw material for the coating. As iswell known in the art, the powder is fed at a controlled rate into athermal spray torch, whereupon the particles are heated (often to beyondthe melting point of the material from which they are made) andentrained within a gas flow, whereupon they impinge upon the surface ofthe substrate to coalesce and form a coating. Any of several thermalspray techniques are suitable for applying the coating. In someembodiments of the present invention, applying the thermal-sprayedcoating comprises applying the coating using at least one of air plasmaspraying, vacuum plasma spraying, and high-velocity oxy-fuel spraying.The thickness of the coating is selected to provide adequate protectionfor the particular environment and desired service life of the substratebeing coated. In certain embodiments, applying the thermal-sprayedcoating comprises applying a coating having a thickness of greater thanabout 20 micrometers. In particular embodiments, applying thethermal-sprayed coating comprises applying a coating having a thicknessin the range from about 100 micrometers to about 1500 micrometers. Tofurther enhance the effectiveness of the protective thermal-sprayedcoating, the method of the present invention, in some embodiments,further comprises applying at least one intermediate layer onto thesubstrate prior to applying the thermal-sprayed coating. The at leastone intermediate layer is applied to the substrate by any of severalmethods, including, for example, any of the thermal spray methods listedabove. In some embodiments, applying the at least one intermediate layercomprises applying at least one layer comprising mullite, and in furtherembodiments, applying the at least one intermediate layer comprisesapplying at least one layer comprising silicon, such as, for example, asilicate compound. Intermediate layers provide enhanced adhesion betweenthe protective coating and the substrate and, in some instances, preventreactions between the substrate and the protective coating.

In order to further enhance the adhesion of the thermal-sprayed coatingto the substrate, the method of the present invention, in certainembodiments, further comprises heat treating the substrate afterapplying the thermal-sprayed coating. Heat-treating the coating providesconversion of the coating to an appropriate crystal structure. In thecase of BSAS, the as-sprayed coating has a crystal structure that is acombination of amorphous and hexacelsian structure. Duringheat-treatment, the coating is transformed to >50% monoclinic celsianphase. The coating gains strength after heat-treatment and the celsianphase has a coefficient of thermal expansion (CTE) that is more similarto the silicon-containing substrate (˜5×10^−6/C) than the hexacelsianphase (˜8×10^6/C). In particular embodiments, heat-treating comprisesheating said substrate to a temperature in the range from about 1200° C.to about 1400° C. for a time in the range from about 15 minutes to about100 hours. In certain instances, the heat treatment is performed inflowing air in order to ensure that oxygen reduction does not occur inthe oxide-based coating.

In order to further exploit the advantages of embodiments of the presentinvention, certain embodiments of the present invention include a methodfor manufacturing an article for use in a high-temperature environment,the method comprising: providing a substrate comprising silicon;selecting a desired vertical crack density for an environmental barriercoating to be deposited on the substrate; providing a powder comprisingbarium strontium aluminosilicate, wherein the powder has a size rangeselected to provide a coating having the desired vertical crack density;and applying a thermal-sprayed coating to the substrate, the coatinghaving the desired vertical crack density, wherein the powder is used asa raw material for the coating.

Embodiments of the present invention also include an article for use inhigh temperature environments, the article comprising a substratecomprising silicon, and an environmental barrier coating (EBC)comprising barium aluminosilicate, wherein the coating comprises avertical crack density of greater than about 4 cracks per linearcentimeter. Although the presence of vertical cracks in the EBC reducesits barrier properties, the reduction may be offset somewhat by theincrease in strain tolerance of the EBC due to the vertical cracks. Incertain applications, such a tradeoff may be desirable, in thatincreased strain tolerance of the EBC may increase the overall lifetimeof the component, despite the somewhat reduced protective capability. Ifthe cracks are narrow and are not held open during exposure to corrosivegases, the protective capability of the coating may not be diminished tothe point of inadequacy. In certain embodiments, the vertical crackdensity is in the range from about 4 cracks per linear centimeter ofcoating to about 12 cracks per linear centimeter of coating.

In certain embodiments, the substrate comprises at least one of aceramic comprising silicon, such as, for example, silicon carbide; and asilicon-containing metal alloy. In particular embodiments, the substratecomprises a fiber-reinforced composite material, such as for example, amaterial comprising a matrix comprising silicon (such as, for example,silicon carbide, silicon nitride, or silicon) and a fiber-reinforcement,such as, for example, carbon fiber or silicon carbide fiber. Suitableexamples of silicon-containing metal alloys include niobium-siliconalloys and molybdenum-silicon alloys. As in the method embodimentsdescribed above, in certain embodiments the substrate comprises acomponent of a gas turbine assembly.

As described above, intermediate layers are often used in EBC systems toenhance the performance of the coating. Accordingly, in certainembodiments of the present invention, the article further comprises atleast one intermediate layer disposed between said substrate and saidenvironmental barrier coating, and in particular embodiments, the atleast one intermediate layer comprises at least one of mullite andsilicon. Other aspects of the article, such as coating thickness, forexample, are consistent with the descriptions above for the method ofthe present invention.

EXAMPLE

The following example is included to describe exemplary embodiments ofthe present invention and should not be considered as limiting theinvention in any way.

Each of a baseline grade BSAS powder, a BSAS class A powder, and a BSASclass B powder was air plasma sprayed according to the parameters inTable 1 onto substrates made of a melt-infiltrated siliconfiber-reinforced silicon/silicon carbide composite.

TABLE 1 Coating layer I II III Powder Si Mullite/BSAS BSAS Powder size(mesh size) −100 + 325 −200 + 325 Coating passes 15 18 20 Ar PrimaryGas, scfh 80 80 80 H Secondary Gas, scfh 5-8 8-11 6-9 Ar Carrier Gas,scfh 10 12 12 Gun current, A 650 700 540 Gun voltage, V 44 49 46 Gunpower, kW 29 34 25 Powder feed rate, kg/hr 1 1 0.5 Spray distance, cm 108 13 Gun speed, mm/s 600 1000 600

Prior to deposition of the BSAS material (coating layer III in Table 1),the substrates were coated with from about 50 micrometers to about 100micrometers of silicon (coating layer I) and from about 50 micrometersto about 100 micrometers of a layer comprising a mixture of BSAS andmullite (coating layer II). After all coatings were deposited, thespecimens were heat treated at about 1250° C. for about 24 hours inflowing air. The specimens were then sectioned and metallographicallyexamined to measure vertical crack density. The results showed a clearrelationship between vertical crack density and powder sizedistribution. The coatings produced using baseline grade powder had thehighest vertical crack density of the three grades tested, at about 7cracks per linear centimeter. Coatings produced using class B powder,with an amount of fine particles intermediate to that of baseline gradeand class A powder, had about 4 cracks per linear centimeter, and thoseproduced using class A powder, with the smallest amount of fineparticles, had about 0 cracks per linear centimeter.

While various embodiments are described herein, it will be appreciatedfrom the specification that various combinations of elements,variations, equivalents, or improvements therein may be made by thoseskilled in the art, and are still within the scope of the invention asdefined in the appended claims.

1. A method for manufacturing an article for use in a high-temperatureenvironment, said method comprising: providing a substrate; selecting adesired vertical crack density for a protective coating to be depositedon said substrate; providing a powder, wherein said powder has aparticle size distribution selected to provide a thermal spray coatinghaving said desired vertical crack density; and applying athermal-sprayed coating to said substrate, said coating having saiddesired vertical crack density, wherein said powder is used as a rawmaterial for said coating; and applying at least one intermediate layeronto said substrate prior to applying said thermal-sprayed coating. 2.The method of claim 1, wherein selecting said desired vertical crackdensity comprises selecting a vertical crack density from about 0 cracksper linear centimeter of coating to about 30 cracks per linearcentimeter of coating.
 3. The method of claim 2, wherein selecting saidvertical crack density comprises selecting a vertical crack density fromabout 0 cracks per linear centimeter of coating to about 12 cracks perlinear centimeter of coating.
 4. The method of claim 3, whereinselecting said vertical crack density comprises selecting a verticalcrack density of about 0 cracks per linear centimeter of coating.
 5. Themethod of claim 1, wherein providing said powder comprises providing apowder having a particle size distribution wherein about 10 volumepercent of said powder comprises particles having diameters less than atenth percentile diameter value in the range from about 25 micrometersto about 50 micrometers, about 50 volume percent of said powdercomprises particles having diameters less than a fiftieth percentilediameter value in the range from about 50 micrometers to about 70micrometers, and about 90 volume percent of said powder comprisesparticles having diameters less than a ninetieth percentile diametervalue in the range from about 85 micrometers to about 105 micrometers.6. The method of claim 5, wherein providing said powder comprisesproviding a powder having a particle size distribution wherein saidtenth percentile diameter value is in the range from about 25micrometers to about 30 micrometers, said fiftieth percentile diametervalue is in the range from about 50 micrometers to about 55 micrometers,and said ninetieth percentile diameter value is in the range from about85 micrometers to about 90 micrometers.
 7. The method of claim 5,wherein providing said powder comprises providing a powder having aparticle size distribution wherein said tenth percentile diameter valueis in said range from about 35 micrometers to about 50 micrometers, saidfiftieth percentile diameter value is in said range from about 55micrometers to about 70 micrometers, and said ninetieth percentilediameter value is in said range from about 85 micrometers to about 105micrometers.
 8. The method of claim 7, wherein providing said powdercomprises providing a powder having a particle size distribution whereinsaid tenth percentile diameter value is in the range from about 35micrometers to about 45 micrometers, said fiftieth percentile diametervalue is in the range from about 55 micrometers to about 65 micrometers,and said ninetieth percentile diameter value is in the range from about85 micrometers to about 90 micrometers.
 9. The method of claim 7,wherein providing said powder comprises providing a powder having aparticle size distribution wherein said tenth percentile diameter valueis in the range from about 45 micrometers to about 50 micrometers, saidfiftieth percentile diameter value is in the range from about 60micrometers to about 70 micrometers, and said ninetieth percentilediameter value is in the range from about 95 micrometers to about 105micrometers.
 10. The method of claim 1, wherein providing said substratecomprises providing a material comprising silicon.
 11. The method ofclaim 10, wherein providing said material comprises providing at leastone of a silicon-containing ceramic and a silicon-containing metalalloy.
 12. The method of claim 11, wherein providing said ceramiccomprises providing a fiber-reinforced composite material.
 13. Themethod of claim 1, wherein providing said substrate further comprisesproviding a component of a gas turbine assembly.
 14. The method of claim1, wherein providing said powder comprises providing a material capableof forming an environmental barrier layer to protect said substrate froma high temperature environment.
 15. The method of claim 14, whereinproviding said powder comprises providing a ceramic material.
 16. Themethod of claim 15, wherein providing said ceramic material comprisesproviding a material comprising barium strontium aluminosilicate. 17.The method of claim 1, wherein applying said thermal-sprayed coatingcomprises applying a coating having a thickness of greater than about 20micrometers.
 18. The method of claim 17, wherein applying saidthermal-sprayed coating comprises applying a coating having a thicknessin the range from about 100 micrometers to about 1500 micrometers. 19.The method of claim 1, wherein applying said at least one intermediatelayer comprises applying at least one layer comprising mullite.
 20. Themethod of claim 1, wherein applying said at least one intermediate layercomprises applying at least one layer comprising silicon.
 21. The methodof claim 1, further comprising heat-treating said substrate afterapplying said thermal-sprayed coating.
 22. The method of claim 21,wherein heat-treating comprises heating said substrate to a temperaturein the range from about 1200° C. to about 1400° C. for a time in therange from about 15 minutes to about 100 hours.
 23. The method of claim1, wherein applying said thermal-sprayed coating comprises applying saidcoating using at least one of air plasma spraying, vacuum plasmaspraying, and high-velocity oxy-fuel spraying.
 24. The method of claim1, wherein providing said powder comprises providing a material capableof forming a thermal barrier layer to protect said substrate from a hightemperature environment.
 25. The method of claim 24, wherein providingsaid powder comprises providing a ceramic powder.
 26. The method ofclaim 25, wherein providing said ceramic powder comprises providing amaterial comprising yttria-stabilized zirconia.
 27. A method formanufacturing an article for use in a high-temperature environment, saidmethod comprising: providing a substrate comprising silicon; selecting adesired vertical crack density for an environmental barrier coating tobe deposited on said substrate; providing a powder comprising bariumstrontium aluminosilicate, wherein said powder has a size range selectedto provide a coating having said desired vertical crack density; andapplying a thermal-sprayed coating to said substrate, said coatinghaving said selected vertical crack density, wherein said powder is usedas a raw material for said coating.